The primary colours of light are red, green, and blue. These colours are also called basic colours of light. the reason for considering red, green and blue as primary colours is that all the other colours are made by mixing primary colours in suitable proportions. An interesting thing to be noted about primary colours is that when red, green and blue colours are mixed together they make white light.

Secondary Colours (Composite Colours)

The colours produced by mixing any two primary colours of light are called secondary colours or composite colours. Magenta, cyan and yellow colours are secondary colours.

Experiment for Formation of Secondary Colours

Take three torches and cover there glasses with red, green and blue cellophane papers, so as to produce red, green and blue light respectively. Now, switch on the torches and project all the three coloured lights on a white screen or wall, so that these coloured light may overlap. Now, you will observe that the area where red and green coloured lights overlap appears yellow. And the area where red and blue coloured lights overlap appears magenta. In the same way the area where blue and green coloured lights overlap gives cyan colour. Also, you will observe that the area where all the coloured lights overlap, appears white. We can also write these results as given below:

Red + Green = Yellow

Red + Blue = Magenta

Blue + Green = Cyan

Complementary Colours

Complementary colours are the two colours, which give white light when mixed together. For example, red and cyan are complementary colours because they produce white light on mixing together. In the same way, blue and yellow, and green and magenta are also complementary colours. The complementary colours can be easily remembered . The colours present exactly opposite to each other in the triangle are complementary colours.

The use of complementary colours is also common in our daily life. The best example of it is the mixing of indigo in lime during white washing of buildings. Actually, with the passage of time the colour of buildings becomes yellowish. Because blue colour is complementary colour of yellow colour so mixing of indigo in lime during white washing keeps the buildings white for a long time.